1. Field of the Invention
The present invention relates to a pacemaker of the type that is operable in a tracking and a non-tracking mode and having an automatic mode switching function for switching the pacemaker into a non-tracking mode of operation in response to the detection of an atrial tachycardia, and including an atrial detector for detecting atrial events, a ventricular detector for detecting ventricular events, an atrial interval determining unit for determining the interval between consecutive atrial events, a comparator for comparing this interval with a predetermined atrial tachycardia limit value for recording a tachy indication if the interval is less than the atrial tachycardia limit value, and a mode switching unit for switching the mode of operation to the non-tracking mode when the number of recorded tachy indications reaches a predetermined tachy count limit.
2. Description of the Prior Art
Patients with paroxysmal atrial tachycardias who require e.g. DDD pacing run the risk of entering into a situation of inappropriate rapid pacing due to tracking of the atrial rhythm. It has been observed that a non-physiologic high ventricular stimulation rate in a tracking mode of operation is the primary source of the suffering of the patient and not the atrial tachycardia in itself. As a rule, more than 50% of patients, which have had periods of atrial tachycardia before the implantation of a pacemaker, will have it again, and more than 30% of pacemaker patients which have not had any suspected atrial tachycardia will get at least one episode of tachycardia during a five year period after implantation. If these patients were paced with a pre-set rate, possibly modulated by a rate-adaptive sensor, they would not have AV synchrony during periods of sinus rhythm and therefore be compromised. Several solutions employing mode switching have therefore been proposed to avoid inappropriate tracking of atrial arrhythmias and to provide tracking of the sinus mode at all other times. Usually such a mode switch of the pacemaker changes the mode of response to atrial sensed events from a tracking to a non-tracking mode. When the atrial rhythm exceeds a predetermined detection rate for a set number of intervals the ventricle will consequently be paced at a predetermined rate and when sinus rhythm resumes, the pacemaker is switched back to an atrial tracking mode.
Thus in U.S. Pat. No. 5,085,215 a metabolic demand driven rate-responsive pacemaker is described, which is switched into a VVI mode, when a maximum atrial tracking rate is reached. In the VVI mode ventricular pacing pulses are generated at a rate which is a function of a metabolic indicator rate and independent of atrial heartbeat sensing.
U.S. Pat. No. 5,514,164 discloses an implantable pacemaker normally operating in a DDD mode and reverting to a modified DDI mode in response to the sensing of atrial depolarization early in the pacing cycle. The DDI mode operates in the same way as the DDD mode except that the atrial signals are not tracked. Hence, detection of P-waves in the DDI mode result in inhibition of atrial output with normal ventricular timing. The DDI mode continues only to the end of the pacing cycle. During the next pacing cycle the DDI mode is initiated again only if a P-wave is again sensed early in the pacing cycle, otherwise DDD pacing continues.
There is a need for improvement of automatic mode switch algorithms known today.
Some known algorithms may be considered slow because of too long a period of confirmation before the mode switch occurs and some systems switch mode incorrectly caused by improper detector design resulting in false indications in conjunction with too short a confirmation period. Some automatic mode switch systems do not have a reliable signal sensing, which may also result in non-detection of some cardiac events. Further, unnecessarily long refractory periods of known systems may give blindness to atrial tachycardia and the time from tachycardia starts until mode switch is very unpredictable and dependent on the tachycardia rate or programmed tachycardia limit.
There does not exist any automatic mode switch design today, which is suitable for all types of patients. A few patients prefer a longer time for the mode switching, typically several seconds, if false mode switchings can be avoided in this way. However, most patients want to avoid any unnecessarily high stimulation rates, even for a few seconds.
A correct signal detection is the first and most important factor for a satisfactory mode switch function. Several types of pacemakers are provided with only one atrial signal detector. When the sensitivity of this detector is adjusted appropriately for detection of normal P-waves, some types of atrial tachycardias will be detected only intermittently. The mode switching then has to be based on “guessings”. Further, during pacemaker implantation and the accompanying follow-up atrial tachycardia signals are most often not obtainable. It is therefore an obvious risk that the physician will choose a sensitive setting, which is too high or too low for atrial tachycardia detection. Often a very high sensitivity is chosen, which together with unsuitable filter designs may result in the detection of R-wave farfields and also interferences, which may influence the normal pacing rate and even give rise to false mode switches.
This detector sensitivity dilemma is one reason for a problematic mode switch function of several existing products.
Thus, to distinguish an atrial tachycardia from an interference or some premature heart signals merely by measurement of atrial intervals and counting the number of such intervals might not be sufficient. As a matter of fact several factors have to be considered.
If farfield R-waves are sensed by the atrial detector the pulse generator timers of the pacemaker will be restarted with restart of refractory periods as well. Ventricular stimulations with accompanying blanking periods are also synchronized to the sensed atrial signal, the stimulation rate being of course limited by the maximum tracking rate. The blanking intervals, which may cover atrial events, are consequently repeated more frequently. All these intervals resulting from a mixture of sensed and stimulated events must be considered when designing an automatic mode switch system and not only the free atrial—atrial signals.
If spontaneous ventricular beats are omitted, three different situations of mixed sensed and stimulated cardiac occurrences are possible during an atrial tachycardia in DDD mode of operation of a pacemaker. These situations are illustrated by symbolic ECG's designated a, b and c in FIG. 1 a), b) and c).
AF denotes an atrial signal, which can be a P-wave, flutter or fibrillation sensed on the atrial electrode. V denotes a ventricular stimulation pulse, and ? denotes the associated blind atrial blanking period, during which an atrial signal may have occurred.
The situation illustrated at a) in FIG. 1 is characterized by one atrial signal AF intervening between two ventricular stimulations V. An atrial signal can be hidden during the atrial blanking ?. Since this situation can be stable during an atrial flutter, it can be hard to distinguish between atrial flutter and a normal situation with a high sinus rate without any hidden signals. Even if not all the atrial signals are hidden during an unstable tachycardia, a sufficient number of atrial signals can disappear such that a pacemaker, the control of which is based on long/short atrial intervals, can be wrongly operated. The only effective solution to this problem is to change the stimulation interval, such that when the time of ventricular stimulation is altered, the atrial blanking interval will not be positioned on half the atrial—atrial signal interval.
The situation according to b) in FIG. 1 with two atrial signals AF between two consecutive ventricular stimulations V will make detection possible for all situations provided that the used pulse generator design does not involve unnecessary refractory periods. The algorithms used for controlling the pacemaker can be more or less effective in situations of coverage of some of the atrial signals by the post ventricular atrial blanking periods ?, because there will be a measured long atrial interval which can counteract the tachycardia determination.
At c) in FIG. 1 a situation is shown with three atrial signals AF appearing between each two consecutive ventricular stimulations V. In this case correct AF detection will be possible, provided that the pulse generator design does not involve unnecessary refractory periods and provided that there is no interference protection, which may be erroneously operated by the detected high rate. For this situation, with a very high atrial rate, there is a higher probability for coverage of some of the atrial signals by the post ventricular atrial blanking periods?. If long atrial counting intervals are used in the algorithms for controlling the pacemaker, a slowing down can occur before the mode switch.
Only at extremely high atrial rates and extreme parameter settings four atrial signals AF may occur in one V—V interval. This case is basically similar to the situation shown at c).
Another important factor, which may give rise to problems, is the consideration time from a detection of a short AF—AF interval until a mode switch takes place. Depending on the signal detector design the needed confirmation time will be shorter or longer for one and the same patient depending on the sequence of heart signals in conjunction with other pacemaker timings, such as refractory periods and stimulation intervals. This means that the mode switch confirmation time very often cannot be predicted.